1. Field of Invention
The present invention relates to a polyolefin film is coated with an undercoat consisting essentially of a mixture of (a) polyvinylidene chloride or a copolymer thereof containing at least 88 percent by weight of vinylidene chloride monomer units and (b) a chlorinated polypropylene or a graft polymer thereof at the weight ratio of a/b in the range between 99/1 and 90/10, and a topcoat consisting essentially of a vinylidene chloride copolymer containing not more than 88 percent by weight of vinylidene chloride monomer units. More particularly, the invention relates to coated polyolefin films which are excellent in the low-temperature heat-sealing property, the barrier property to oxygen gas and water vapor, the adhesive cellophane tape peel strength of the coating and the transparency.
2. Description of Prior Arts
Biaxially stretched films of isotactic polypropylene now attract attention in the art because of their high mechanical properties and excellent transparency. However, these films are inferior in the oxygen gas barrier property and since they are highly crystalline and are biaxially stretched, thermal shrinkage is caused at high temperatures and the heat-sealing property is quite insufficient.
As means for improving the oxygen gas barrier property and the heat-sealing property, there has been proposed a process comprising coating such polypropylene film with a mixture of a vinylidene chloride type resin and a chlorinated polypropylene resin or a chlorinated polypropylene resin grafted with a vinyl monomer such as styrene (see Japanese Patent Publications Nos. 16316/68, 26085/68, 277/70 and 8400/72). In this method, however, films having a good heat-sealing property cannot be obtained, because a vinylidene chloride resin having a relatively high vinylidene chloride content is used in order to improve the oxygen gas barrier property.
As means for improving the low-temperature heat-sealing property, there have been adopted a method in which a vinylidene chloride type resin having a vinylidene chloride content reduced to some extent is employed and a method in which a third component having a low melting point is incorporated. According to these methods, a good low-temperature heat-sealing property can be attained, but the oxygen gas barrier property is insufficient.